1. Field of the Invention
This invention relates to the preparation of polymer foams and is more particularly concerned with novel catalyst combinations for the trimerization of polyisocyanates to polyisocyanurates and their utilization in the preparation of polyisocyanurate foams.
2. Description of the Prior Art
Rigid polyisocyanurate foams having high resistance to flame and heat as well as excellent thermal insulating capacity are known in the art. The prior art discloses methods for preparing such foams by reacting an organic polyisocyanate with a trimerizing catalyst in the presence of a blowing agent, and a minor proportion (usually less than 0.5 equivalent per equivalent of polyisocyanate) of a polyol; see for example U.S. Pat. Nos. 3,516,950, 3,580,868, 3,620,986, 3,625,872, and 3,725,319. The process described in U.S. Pat. No. 3,745,133 discloses the use of a combination of an epoxide and a tertiary amine as cocatalysts.
It will be recognized by one skilled in the art that the method referred to hereinabove requires the foaming process in conjunction with at least two polymer forming reactions; namely, the isocyanurate formation arising from the homopolymerization of the isocyanate employed, and the minor amount of polyurethane formation arising from the reaction of a polyol with the isocyanate. A drawback of the method is the variation in rate of the two polymerization reactions. The polyurethane formation initiates before the trimerization formation thus giving two separate rise steps in the overall foam rise profile.
When foams are prepared by high speed mixing machinery and dispensed on a conveyor line, the two-step rise profile causes "undercutting". The latter term is well recognized by those skilled in the foam art and refers to the movement of fresh, liquid reactant running under foam that is in the process of rising and has not itself reached a gel point. When this occurs, areas of unequal rising and gelling within a given location in a foam bun cause internal stresses which result in splits on the bun surface, or imperfections, either of which can render large sections of the bun useless and are economically unacceptable.
Additionally, a foam bun made by the method referred to hereinabove can be made to a maximum height of only 18 inches. It is well recognized by those skilled in the art that the larger the bun size prepared, the more favorable the economics of manufacture. In yet a further disadvantage arising from the two-step rise profile, the angle of the conveyor line to the horizontal must be greater than for a single rise profile, in order to partially accommodate the problems arising from the double rise. Increasing conveyor angles in foam bun line processing introduces additional difficulties ranging from greater space requirements for the machinery, to foam processing difficulties.
We have now found that it is possible to prepare polyisocyanurate foams by processes, in particular, by continuous bun stock operation, which do not have the disadvantages to be found in the prior art methods. Such foams can be processed on standard high speed mixing machines and dispensed on conveyor lines at normal angles. They can be easily formed to a 24 inch bun height and have an essentially single rise profile thereby eliminating the problem of undercutting. In addition, we have also found that the foams made in accordance with the present invention possess unexpectedly useful properties in that they generate as much as 55% less smoke during combustion tests than the closely related prior art foams made with previously known catalyst systems.